Optimization of Rolling Conditions in Nb Microalloyed Steels Processed by Thin Slab Casting and Direct Rolling Route: Processing Maps

Optimization of Rolling
Conditions in Nb Microalloyed
Steels Processed by Thin Slab
Casting and Direct Rolling Route:
Processing Maps

P. Uranga, A.I. Fernández, B. López and J.M. Rodriguez-Ibabe
CEIT and tecnun (University of Navarra)
Donostia-San Sebastián
Basque Country, Spain

Microalloying for New Steel Processes and Applications,
September 7-9, 2005, Donostia-San Sebastián

Objective
• Definition of Optimal Processing Conditions
for different grades under a wide variety of
Industrial Rolling Conditions

• Special attention to:

– Avoidance of microstructural heterogeneities
in thick plates and high levels of microalloying
additions

– Conditioning of austenite structure prior to
transformation

Procedure
• Classical approach:
– Not enough to predict heterogeneities
• New model:
– Particular characteristics of TSDR Technology
• Initial As-cast Structure
• Specific Thermomechanical Deformation
Route
• Processing Maps:
– Suitable for visual understanding

Model
Description
• Main difference:
– Expansion of Classical Microstructural
Modeling to Grain Size Distributions
• Input:
– Grain Size Distribution measured in a real
Thin Slab
• Output:
– Recrystallized and Unrecrystallized Grain
Size histograms and Retained Strain

[fv] i 3-D

Frequency
[d0] i D
1st rolling
pass
1 … i1th interval … n1

Rex Unrex

. .
.
.
Frequency
[X ]i , [d r ]i [du ]i , [1− X ]i , [ε r ]i .
.

Log-normal
Distribution

pth rolling
[drex] i D
pass
1 … kpth interval … np

. Rex . Unrex .
. . .
. . .

Recrystallized Fraction Final Microstructure Unrecrystallized Fraction
Histograms
Area Fraction

Area Fraction

Grain Size Grain Size

Rolling Simulations

• 0.035% Nb Microalloyed Steel
(0.06%C, 0.008%N, 1.1%Mn)
• Initial thickness: 55 mm
• Final thicknesses: 1.5 to 12.65 mm
• Rolling entry temperatures: 1040 to 1100ºC
• Interpass times: function of strain-rates
• Final cooling rate: 20ºC/s to 800ºC

Rolling
Schedules

e (mm) 4 10
ΔT
Pass no. Strain Rate tip Strain Rate tip
1 0.7 5 6 0.5 5 6 35
2 0.7 10 4 0.5 10 4 30
3 0.55 15 3 0.45 15 5 30
4 0.45 30 2.1 ⎯ ⎯ ⎯ 30
5 0.35 50 1.8 0.3 20 2.7 30
6 0.25 70 0.25 25 ⎯

• Large reductions during first passes

Simulation
Results
• Final Austenite Microstructure
0.5 0.5
e = 4 mm e = 10 mm
Dmean= 9 μm Ti = 1040ºC Ti = 1040°C
Ti = 1100ºC Ti = 1100ºC
0.4 Ti = 1100ºC 0.4 Ti = 1100ºC
Dmean= 16 μm
Dmean= 15 μm
Area Fraction

Area Fraction
0.3 0.3

Dmean= 21 μm
0.2 0.2

0.1 0.1

0.0 0.0
10 30 50 70 90 More 10 30 50 70 90 More
Austenite Grain Size (μm) Austenite Grain Size (μm)

Parameter
Definition
• Histograms not very useful when analyzing
wide spread of conditions and/or materials
• General parameters defined:
– Dmean
– Dmax
– Dc (10% of the volume fraction of grains have
a bigger size than Dc)
– ZD (=Dmax/Dmean)

Parameter Evolution
Entry Rolling Temperature: 1040ºC
1600 14 1600 14
e = 4 mm
Ti = 1040ºC
1400 12 1400 12
Dmean
1200 Dc 1200
Dmax 10 e = 10 mm 10

Grain Size (μm)
Grain Size (μm)

ZD Parameter
ZD Parameter
1000 ZD 1000 Ti = 1040ºC
8 Dmean 8
800 800 Dc
6 Dmax 6
600 600 ZD
4 4
400 400

200 2 200 2

0 0 0 0
1 2 3 4 5 6 1 2 3 4 5 6
Interstand Interstand

• e = 4 mm: Homogeneous Structure. ZD < 8

• e = 10 mm: Microstructural Heterogeneities. ZD > 8

Effect of the
final gauge thickness
Final thickness (mm)
12.65 10 7 6 4 3 2 1.5
30
• Dmean a) Dmean

Mean Austenite Grain Size (μm)
25 Ti = 1100°C
– Strain ↑: Dmean↓ Ti = 1040°C
20
– Ti ↑: Dmean ↑
15
• Heterogeneities
unrevealed 10

5

0
1 2 3 4 5
Total Strain

Effect of the
12.65 10 7 6 4 3 2 1.5
80
• Dc b) Dc

Dc Austenite Grain Size (μm)
Ti = 1100°C
– Strain ↑: Dc↓ 60 Ti = 1040°C

– Ti ↑: Dc ↑
40
• 1040ºC: sharp
increase for low 20
strains and low Ti
0
1 2 3 4 5
Total Strain

Effect of the
12.65 10 7 6 4 3 2 1.5
25
• ZD c) ZD
Ti = 1100°C
– Ti ↑: ZD constant 20
Ti = 1040°C

– Strain ↓ and Ti ↓ :
ZD Parameter
15
ZD ↑
• Heterogeneities 10

revealed 5

0
1 2 3 4 5
Total Strain

Processing Maps
Final Gauge Thickness (mm)
12.65 10 7 6 4 3 2 1.5

1100 40

60
Dc
Rolling Entry Temperature (ºC)

1090

50

40
35
35
70

1080

30
60

40
1070
Op 35 30
tim
50

um
1060 Pr
oc
e ss 25
Residual ing
1050 unrefined Zo
60

ne
30
40

as-cast
35

grains
50

20
25

1040
2 2.5 3 3.5 4
Total Strain

• Dc isoclines combined with Processing Regions

Processing Maps
Final Gauge Thickness (mm)
12.65 10 7 6 4 3 2 1.5

1100
0.2
Retained

Strain
1090
0.2

1080 0.3

0.2
0.4
1070 Op 0.3
tim 0.3
um
0.4

Pr
1060 oc
es 0.4
0.5

0.5
Residual sin
unrefined
g Zo
0.
3
1050
0.
6 as-cast ne 0.5
0.4
grains 0.5
0. .7 0.
0.6 0 0. 8
0.6

7 6
1040
2 2.5 3 3.5 4
Total Strain

• Retained strain isoclines combined with Processing
Regions

Processing Maps
Final Gauge Thickness (mm) Final Gauge Thickness (mm)
12.65 10 7 6 4 3 2 1.5 12.65 10 7 6 4 3 2 1.5

1100 40 1100
60

0.2
Dc Retained


Strain
50

1090 1090
40
35
35 0.2
70

0.3
1080 1080

30
60

40

0.2
0.4
1070
Op 1070 Op
35

0.3
30 tim
tim
50

0.3
um um

0.4
Pr Pr
1060
oc 1060 oc
es 0.4

0.5
es 0.5
sin
Residual sin 25 Residual
g 0.
1050 unrefined g Zo unrefined Zo 3
1050
ne
60

0. 4
ne
30
40

as-cast 0. as-cast
6 0.5
35

grains grains 0.5
0. 7 0.
0.6 0. 0.8
50

20

0.6
7
25

6
1040 1040
2 2.5 3 3.5 4 2 2.5 3 3.5 4
Total Strain Total Strain

• e ≤ 6-7 mm: Dc ↔ Ti relationship.
Homogeneous structure.

• e > 6-7 mm: Dc independent from Ti.
Minimum Ti required to avoid heterogeneities.

Conclusions
• The microstructural evolution during hot rolling of coarse grain sized
austenite has been modeled considering all the microstructural reactions
that can take place during an industrial TSDR production of a Nb
microalloyed steel.

• The model is able to predict possible heterogeneities present in the final
austenite microstructure before transformation.

• Based on the results obtained with the model, processing maps have been
drawn for a 0.035% Nb microalloyed steel, considering the following
parameters: initial rolling temperature, total reduction and rolling schedule.

• The influence of the previous parameters on the microstructural refinement
and homogeneity has been evaluated. The processing maps have revealed
as a very useful tool to define optimum processing conditions in order to
exploit all the benefices of the Nb microalloying in thin slab direct rolling

Acknowledgments
• Basque Government
• CICYT (MAT2002-01174 project)

• Materials Department - Thermomechanical
Treatments Group (CEIT)

• CEIT and tecnun
• Carnegie Mellon University and Professor
A.W. Cramb

Grain Size Distribution
in an Industrial Thin Slab
30
Center
25 Near Surface
Mixed
Frequency (%)

20
Columnar-Equiaxed
15
Structure
10

5

0
0 500 1000 1500 2000 2500 3000
Grain Size (μm)

Dγ > 2000 μm → Volume Fraction > 20%

Optimization of Rolling Conditions in Nb Microalloyed Steels Processed by Thin Slab Casting and Direct Rolling Route: Processing Maps

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Optimization of Rolling Conditions in Nb Microalloyed Steels Processed by Thin Slab Casting and Direct Rolling Route: Processing Maps